What Is a Stale Block?

A block that was successfully mined but not included on the current longest blockchain, usually because another block at the same height was added to the chain first. A stale block can be the result of network latency and causes the network to split temporarily into two competing blockchains. Miners resolve the split by continuing to mine new blocks to the chain, considered the valid blockchain. This eventually causes other miners to follow and treat this chain as the true chain. 

The stale block is part of the chain that is no longer being mined and is thus considered invalid. Moreover, the mining reward attached to the stale block is also invalid and cannot be spent. The transactions from the stale block return to the mempool and are mined in subsequent blocks. 

What Causes Stale Blocks?

Bitcoin has a very low number of stale blocks per year. For example, in 2019, only two stale blocks were mined on Bitcoin, thanks to the low latency between mining pools. Latency thus has a considerable effect on the emergence of stale blocks.

Miners that mine a block broadcast it to the nodes closest to them, which pass it on to other nodes in proximity and so on. The transmission of data is not instantaneous, which opens up the possibility of another miner finding the solution to the same block at the same time. 

For example, if one miner is in North America and another is in Australia, they may both find the solution to a block at the same height at the same time. Each miner broadcasts their solution to the nodes closest to them. The stale block eventually becomes apparent as miners see both blocks. The network then has to settle on a block by majority decision, with the longer blockchain being mined and the transactions from the stale block returning to the mempool.

Stale Blocks on Ethereum and Other Blockchains

Before Ethereum's switch to proof-of-stake, stale blocks were also possible on Ethereum. They were called "uncle blocks" and miners could still earn a reward from them, albeit less than the usual mining reward. Other proof-of-work chains can also have stale blocks, which are generally more common than on Bitcoin.